1. Field of the Invention
The present invention relates to a wavelength multiplexed light source and a wavelength multiplexed light source system for wavelength division multiplexing communication. For example, the present invention relates to a wavelength multiplexed light source and a wavelength multiplexed light source system in which wavelengths are multiplexed by a coupler using a multimode interference within a multimode waveguide.
2. Related Art of the Invention
The recent increased demand for communication has remarkably developed a wavelength division multiplexing communication which achieves an increased capacity of transmission without establishing a new transmission path.
A wavelength multiplexed light source is a device used for the wavelength division multiplexing communication. For example, Japanese Patent Laid-Open No. 2000-77756 proposes a wavelength multiplexed light source which includes a 1×N waveguide splitter using MMI (multimode interference) effect, an N number of semiconductor optical amplifiers that are connected to each access wave guide of the 1×N waveguide splitter, and an N number of waveguides having a function to select a wavelength.
FIG. 12 is a diagram showing a conventional wavelength multiplexed light source which is proposed in Japanese Patent Laid-Open No. 2000-77756. FIG. 12 shows a case where the number N is equal to 4.
An output waveguide 401 is connected with a waveguide splitter 402 having a 1×4 type MMI structure. The waveguide splitter 402 has access wave guides which are connected to semiconductor optical amplifiers 4041, 4042, 4043 and 4044 having curved waveguides, respectively, and the semiconductor optical amplifiers 4041, 4042, 4043 and 4044 are connected to fiber gratings 4071, 4072, 4073 and 4074 at one end thereof in which lights have different central wavelengths λ1, λ2, λ3 and λ4 respectively.
The combination of the semiconductor optical amplifiers 4041, 4042, 4043 and 4044 and the fiber gratings 4071, 4072, 4073 and 4074 forms a resonance structure for emitting a laser beam.
The above structure provides an improved wavelength selectivity which is one of the wavelength dependences in space domain.
As a wavelength multiplexed light source having another structure, for example, Japanese Patent Laid-Open No. 2001-166160 proposes an optical coupler for minimizing optical loss in each wavelength, which includes a 1×N waveguide splitter using MMI effect, a plurality of single mode waveguides that are optically connected to the waveguide splitters for individually transmitting different wavelengths therethrough, and multimode waveguides that are interposed between the waveguide splitters and the single mode waveguides having a predetermined length depending on the wavelength respectively.
FIG. 13 is a two-dimensional diagram showing a structure of a conventional optical coupler proposed in Japanese Patent Laid-Open No. 2001-166160, FIG. 13 shows a case where the number N is equal to 4.
The optical coupler includes a multimode optical interference region 120A which is formed with an incident end surface 1202 and an exit end surface 1201, opposite to the incident end surface 1202, and has a physical width Wm.
The incident end surface 1202 are optically connected with a plurality of incident-side single mode optical waveguides 120C1, 120C2, 120C3 and 120C4 for transmitting optical signals which have wavelengths of λ1, λ2, λ3 and λ4 (where λ1>λ2>λ3>λ4) respectively, and between the optical waveguides 120C2, 120C3 and 120C4 and the incident end surface 1202, extending sections, 120E2, 120E3 and 120E4 are interposed respectively.
The exit end surface 1201 is connected with a single mode optical waveguide 120B for transmitting the wavelength-multiplexed optical signals having wavelengths of λ1, λ2, λ3 and λ4.
In this case, the multimode optical interference region 120A has a length of Lmmi which is set to cause the incident light having a wavelength λ1 from the optical waveguide 120C1 to reach the exit end surface 1201 with a minimum optical loss. Also, each of the extending sections 120E2, 120E3 and 120E4 has a length of Lext which is set so that the length of Lmmi+Lext causes each of the lights having the wavelength λ2, λ3 and λ4 to reach the exit end surface 1201 with a minimum optical loss.
Each of the extending sections 120E2, 120E3 and 120E4 has a width larger than those of the single mode optical waveguides 120C2, 120C3 and 120C4, and form a multimode optical waveguide similar to the multimode optical interference region 120A. As a result, when the optical signals enter the extending sections 120E2, 120E3 and 120E4, the optical signals guided through the single mode optical waveguides 120C2, 120C3 and 120C4 are dispersed as shown by the broken lines in FIG. 13, and are incident to the multimode optical interference region 120A.
After entering the multimode optical interference region 120A, the optical signals produce a multimode interference in the multimode optical interference region 120A. Each of the extending sections 120E2, 120E3 and 120E4 has a width larger than those of the single mode optical waveguides 120C2, 120C3 and 120C4 so that the dispersion of the optical signals entering the multimode optical interference region 120A is not disturbed, which allows each of the optical signals having the wavelength λ1, λ2, λ3 and λ4 to have a strong light intensity due to self-imaging effect at the portion of the exit end surface 1201 where the single mode optical waveguide 120B is connected.
The above structure provides an improved optical coupling condition which is one of the wavelength dependences in space domain.
The increased number of multi-wavelengths due to the recent increased demand for communication causes the N number of 1×N waveguide splitters to be increased, including a width of the 1×N waveguide splitters, resulting in that the modal dispersion along the 1×N waveguide splitters is steadily on the increase.
This trend is likely to continue, and the increased modal dispersion along a multimode waveguide may cause a performance degradation due to the wavelength dependence of a modal group velocity dispersion (a modal dispersion which is dominated by time domain). In addition to each condition based on the wavelength dependence of modal dispersion (a wavelength dependence of self-imaging effect) which is dominated by a space domain, it is a desirable to improve the wavelength dependence of modal dispersion, as disclosed, for example, in the above Japanese Patent Laid-Open No. 2000-77756 and Japanese Patent Laid-Open No. 2001-166160.
The problem of performance degradation due to the wavelength dependence of modal group velocity dispersion will be explained below.
The wavelength dependence of a modal group velocity dispersion is one of the characteristics of light that a light of a longer wavelength has a larger group velocity dispersion than a light of a relatively shorter wavelength.
The output pulses of a light of a longer wavelength, having a larger group velocity dispersion which contain pulse groups in eigenmodes that are separated on a time-axis by the group velocity dispersion, are more extensively broadened than those of a light of a shorter wavelength having a smaller group velocity dispersion. Therefore, a large variation in the output pulse widths may supposed to occur in the entire optical signals having different wavelengths which are used in the wavelength multiplexed light source.
The performance degradation due to the wavelength dependence of a modal group velocity dispersion means the efficiency degradation of wavelength multiplexing transmission which is caused by the variation in the output pulse widths of optical signals having different wavelength in the wavelength multiplexed light source.
That is, at the output end of a wavelength multiplexed light source, there is a difference in the transmitting capacities and transmission distances between the optical signal of the shortest wavelength, which causes the smallest output pulse spreading, and the optical signal of the longest wavelength, which causes the largest output pulse spreading. The transmission capacity and transmission distance of the optical signal of the longest wavelength have the largest output pulse spreading, which results in the lowest transmission efficiency, dominating the transmission capacity and transmission distance of the wavelength multiplexing transmission. This is because any combination with a transmission capacity and a transmission distance of another optical signal having another wavelength may make the output pulse spreading of the optical signal of a longer wavelength than such another wavelength indistinguishable, which leads to a transmission error.
As described above, the variation in the output pulse widths of the optical signals having different wavelengths disturbs the efficiency of a wavelength multiplexing transmission.